platform/surface: aggregator: make ssam_bus_type constant and static

Now that the driver core can properly handle constant struct bus_type,
move the ssam_bus_type variable to be a constant structure as well,
placing it into read-only memory which can not be modified at runtime.

It's also never used outside of
drivers/platform/surface/aggregator/bus.c so make it static and don't
export it as no one is using it.

Cc: Maximilian Luz <[e-mail]>
Cc: Hans de Goede <[e-mail]>
Cc: Ilpo Järvinen <[e-mail]>
Cc: <[e-mail]>
Reviewed-by: Maximilian Luz <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>
Link: https://lore.kernel.org/r/2023121957-tapered-upswing-8326@gregkh
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/platform/surface/aggregator/bus.c

···
 };
 ATTRIBUTE_GROUPS(ssam_device);
 
+static const struct bus_type ssam_bus_type;
+
 static int ssam_device_uevent(const struct device *dev, struct kobj_uevent_env *env)
 {
 	const struct ssam_device *sdev = to_ssam_device(dev);
···
 		sdrv->remove(to_ssam_device(dev));
 }
 
-struct bus_type ssam_bus_type = {
+static const struct bus_type ssam_bus_type = {
 	.name   = "surface_aggregator",
 	.match  = ssam_bus_match,
 	.probe  = ssam_bus_probe,
 	.remove = ssam_bus_remove,
 };
-EXPORT_SYMBOL_GPL(ssam_bus_type);
 
 /**
  * __ssam_device_driver_register() - Register a SSAM client device driver.

include/linux/surface_aggregator/device.h

···
 
 #ifdef CONFIG_SURFACE_AGGREGATOR_BUS
 
-extern struct bus_type ssam_bus_type;
 extern const struct device_type ssam_device_type;
 
 /**